A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. To provide a uniform distance between a read/write head and the surface of a magnetic-recording disk, an actuator relies on air pressure inside the hard drive enclosure to support the read/write heads at the proper distance away from the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. That is, the air pulled along by a spinning magnetic-recording disk forces the head away from the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away.
It is desirable, for a variety of reasons, to maintain a constant or approximately constant distance between the read/write head and the surface of the magnetic-recording disk to ensure proper operation of the read/write head. If the distance between a read/write head and the surface of a magnetic-recording disk fluctuates, then the strength of the magnetic dipole field between the read/write head and the surface of the magnetic-recording disk will also fluctuate, which may cause problems in reading data from or writing data to the magnetic-recording disk.